[POWERPC] Stop the TOC overflowing for large builds

We were using -mno-minimal-toc on everything in arch/powerpc/kernel,
which means that all the functions in there were putting all their
TOC entries in the top-level TOC, and it was overflowing on an
allyesconfig build.  For various reasons, prom_init.c does need
-mno-minimal-toc, but the other .c files in there can use sub-TOCs
quite happily.  This change is sufficient for now to stop the TOC
overflowing; other directories under arch/powerpc also use
-mno-minimal-toc and could also be changed later if necessary.

Lmbench runs with and without this patch showed no significant speed
differences.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>